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ThermoelectricHAND BOO K
Product Information
Assembly Information
Performance and Properties
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Table of Contents
Product Information
Introduction to Thermoelectrics ........................................................................................................................................................1
Structure and Function .....................................................................................................................................................................2
Temperature Control ........................................................................................................................................................................4
Parameters Required for Device Selection ........................................................................................................................................6
Sealant Options................................................................................................................................................................................7
Thermoelectric Arrays .......................................................................................................................................................................7
Design/Selection Checklist ...............................................................................................................................................................8
Thermoelectric Multistage (Cascade) Modules ................................................................................................................................9
Typical Device Performance. .............................................................................................................................................................9
Assembly Information
Assembly Tips .................................................................................................................................................................................10
Procedure For Assembling Lapped Modules To Heat Exchangers ...................................................................................................11
Procedure For Assembling Solderable Modules To Heat Exchangers ..............................................................................................12
Performance and Properties
Device Performance Formulae ........................................................................................................................................................13
Heat Transfer Formulae ..................................................................................................................................................................14Typical Properties of Materials (@ 21C) .......................................................................................................................................15
Reliability & Mean Time Between Failures (MTBF) .....................................................................................................................................16
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Introduction to ThermoelectricsSolid state heat pumps have been known since the discovery of
the Peltier effect in 1834. The devices became commercially avail-
able in the 60s with the development of advanced semiconductor
thermocouple materials in combination with ceramics substrates.
Thermoelectric modules (TEMs) are solid-state heat pumps that
require a heat exchanger to dissipate heat utilizing the Peltier Effect.During operation, DC current ows through the TEM to create heat
transfer and a temperature differential across the ceramic surfaces,
causing one side of the TEM to be cold, while the other side is hot.
A standard single-stage TEM can achieve temperature differentials
of up to 70C. However, modern growth and processing methods of
semiconductor materials are exceeding this limitation.
TEMs have several advantages over alternate cooling technologies.
They have no moving parts, so the solid state construction results in
high reliability. TEMs can cool devices down to well below ambient.
Colder temperatures can be achieved, down to minus 100C, by
using a multistage thermoelectric module in a vacuum environment.Thermoelectrics are able to heat and cool by simply reversing the
polarity, which changes the direction of heat transfer. This allows
temperature control to be very precise, where up to 0.01C can be
maintained under steady-state conditions. In heating mode TEMs
are much more efcient than conventional resistant heaters because
they generate heat from the input power supplied plus additional
heat generated by the heat pumping action that occurs.
A typical TEM measures 30 mm x 30 mm x 3.6 mm. Their geomet-
ric footprints are small as they vary from 2 x 2 mms to 62 x 62
mms and are light in weight. This makes thermoelectrics ideal for
applications with tight geometric space constraints or low weight
requirements when compared too much larger cooling technologies,
such as conventional compressor-based systems. TEMs can also be
used as a power generator converting waste heat into energy as
small DC power sources in remote locations.
When should you use thermoelectrics?
Thermoelectrics are ideal for applications that require active cooling
to below ambient and have cooling capacity requirements of up
to 600 Watts. A design engineer should consider them when the
system design criteria includes such factors as precise temperature
control, high reliability, compact geometry constraints, low weight
and environmental requirements. These products are ideal for many
of the consumer, food & beverage, medical, telecom, photonics and
industrial applications requiring thermal management.
ThermoelectricModules availablefrom Laird Technologies
CP Seriesoffer reliable cooling capacity in the range of 10 to 100
watts. They have a wide product breadth that is available in numer-
ous heat pumping capacities, geometric shapes, and input power
ranges. These modules are designed for higher current and larger
heat pumping applications with a maximum operating temperatureof 80C.
OptoTEC Serieshave a geometric footprint less than 13x13 mm
and are used in applications that have lower cooling requirements
of less than 10 watts. These modules offer several surface nishing
options, such as metallization or pre-tinning to allow for soldering
between TEM and mating conduction surfaces.
MS Seriesoffer the highest temperature differential, (T).
Each stage is stacked one on top of another, creating a multistage
module. Available in numerous temperature differentials and
geometric shapes, these modules are designed for lower heat
pumping applications.
ThermaTEC Seriesare designed to operate in thermal cycling
conditions that require reliable performance in both heating and
cooling mode (reverse polarity). Thermal stresses generated in these
applications will cause standard modules to fatigue over time. These
modules are designed for higher current and higher heat pumping
applications with a maximum operating temperature of 175C
UltraTEC Seriesoffer the highest heat pumping capacity within
a surface area. Heat pumping densities of up to 14 W/cm2, or twice
as high as standard modules, can be achieved. The cooling capacity
can range from 100 to 300 watts. TEMs are also ideal for applica-
tions that require low temperature differentials and high coefcientof performance (COP).
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Structure and FunctionSince thermoelectric cooling systems are most often compared
to conventional systems, perhaps the best way to show the
differences in the two refrigeration methods is to describe the
systems themselves.
A conventional cooling system contains three fundamental parts -the evaporator, compressor and condenser. The evaporator or cold
section is the part where the pressurized refrigerant is allowed to
expand, boil and evaporate. During this change of state from liquid
to gas, energy (heat) is absorbed. The compressor acts as the refrig-
erant pump and recompresses the gas to a liquid. The condenser
expels the heat absorbed at the evaporator plus the heat produced
during compression, into the environment or ambient.
A thermoelectric has analogous parts. At the cold junction, energy
(heat) is absorbed by electrons as they pass from a low energy
level in the p-type semiconductor element, to a higher energy level
in the n-type semiconductor element. The power supply provides
the energy to move the electrons through the system. At the hot
junction, energy is expelled to a heat sink as electrons move from a
high energy level element (n-type) to a lower energy level element
(p-type).
Thermoelectric Modules (TEMs) are heat pumps solid state devices
without moving parts, uids or gasses. The basic laws of thermody-
namics apply to these devices just as they do to conventional heat
pumps, absorption refrigerators and other devices involving the
transfer of heat energy.
An analogy often used to help comprehend a thermoelectric cool-ing system is that of a standard thermocouple used to measure
temperature. Thermocouples of this type are made by connecting
two wires of dissimilar metal, typically copper/constantan, in such
a manner so that two junctions are formed. One junction is kept at
some reference temperatures the other is attached to the control
device measurement. The system is used when the circuit is opened
at some point and the generated voltage is measured. Reversing
this train of thought, imagine a pair of xed junctions into which
electrical energy is applied causing one junction to become cold
while the other becomes hot.
Thermoelectric cooling couples (Fig. 1) are made from two elements
of semiconductor, primarily Bismuth Telluride, heavily doped to
create either an excess (n-type) or deciency (p-type) of electrons.
Heat absorbed at the cold junction is pumped to the hot junction at
a rate proportional to current passing through the circuit and the
number of couples.
In practical use, couples are combined in a module (Fig. 2) where
they are connected electrically in series, and thermally in parallel.
Normally a TEM is the smallest component commercially available.
TEMs are available in a great variety of sizes, shapes, operating
currents, operating voltages and ranges of heat pumping capacity.
The trend, however, is toward a larger number of couples operatingat lower currents. The user can select the quantity, size or capacity
of the module to t the exact requirement without paying for
excess power.
There is usually a need to use thermoelectrics instead of other
forms of cooling. The need may be a special consideration of size,
space, weight, efciency, reliability or environmental conditions
such as operating in a vacuum.
Once it has been decided that thermoelectrics are to be considered,
the next task is to select the thermoelectric(s) that will satisfy the
particular set of requirements. Three specic system parametersmust be determined before device selection can begin.
These are:
Tc Cold Surface Temperature
Th Hot Surface Temperature
Qc The amount of heat to be absorbed at the
Cold Surface of the TEM
Figure 1: Cross Section of a typical TE Couple
Figure 2: Typical TE Module Assembly
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In most cases, the cold surface temperature is usually given as part
of the problem that is to say that some object(s) is to be cooled to
some temperature. Generally, if the object to be cooled is in direct
intimate contact with the cold surface of the thermoelectric, the
desired temperature of the object can be considered the tempera-
ture of the cold surface of the TEM (Tc). There are situations where
the object to be cooled is not in intimate contact with the cold sur-face of the TEM, such as volume cooling where a heat exchanger is
required on the cold surface of the TEM. When this type of system is
employed, the cold surface of the TEM (Tc) may need to be several
degrees colder than the ultimate desired object temperature.
The Hot Surface Temperature is dened by two major parameters:
1) The temperature of the ambient environment to which
the heat is being rejected.
2) The efciency of the heat exchanger that is between the
hot surface of the TEM and the ambient environment.
These two temperatures (Tc & Th) and the difference betweenthem (T) are very important parameters and therefore must be
accurately determined if the design is to operate as desired.
Figure 3 represents a typical temperature prole across a
thermoelectric system.
The third and often most difcult parameter to accurately quantify is
the amount of heat to be removed or absorbed by the cold surface
of the TEM, (Qc). All thermal loads to the TEM must be considered.
These thermal loads include, but are not limited to, the active heat
load (I2R) from the electronic device to be cooled and passive heat
load where heat loss can occur through any object in contact with
ambient environment (i.e. electrical leads, insulation, air or gas sur-
rounding objects, mechanical fasteners, etc.). In some cases radiant
heat effects must also be considered.
Single stage thermoelectric modules are capable of producing a no
load temperature differential of approximately 70C. Temperature
differentials greater than this can be achieved by stacking one
thermoelectric on top of another. This practice is often referred to as
Cascading. The design of a cascaded device is much more complex
than that of a single stage device, and is beyond the scope of these
notes. Should a cascaded device be required, design assistance canbe provided by Laird Technologies Engineers.
Once the three basic parameters have been quantied, the selection
process for a particular module or array of TEMs may begin. Some
common heat transfer equations are attached for help in quantify-
ing Qc & Th.
There are many different modules or sets of modules that could
be used for any specic application. One additional criteria
that is often used to pick the best module(s) is Coefcient of
Performance (COP). COP is dened as the heat absorbed at the cold
junction, divided by the input power (Qc / P). The maximum COP
case has the advantages of minimum input power and therefore,
minimum total heat to be rejected by the heat exchanger (Qh =
Qc + P). These advantages come at a cost, which in this case is the
additional or larger TEM required to operate at COP maximum. It
naturally follows that the major advantage of the minimum COP
case is the lowest initial cost.Figure 3: Typical Temperature Relationship in a TEC
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Temperature ControlWhen designing a thermoelectric system power supplies, tempera-
ture controllers, and temperature sensors are components that also
require careful consideration.
Thermoelectric devices require a DC power source to operate. The
power supply output should be matched to the operational voltageof the thermoelectric modules and fans. Do not operate thermo-
electric devices above the specied maximum voltage. Doing so
will degrade the operational performance of the TEMs. The power
supply should also have a small ripple voltage (maximum of 10%
of full output). Ripple voltage is a uctuation of the power supply
output voltage and therefore is an AC component of the DC power
source. AC power will degrade the operational performance of the
TEMs. The degradation in performance due to ripple voltage can be
approximated by:
Temperature control can be accomplished by using one of two con-
trol methods: Open Loop (manual) and Closed Loop (automatic).
In the Open Loop method, an operator adjusts the output of the
power supply to achieve and maintain a steady temperature. In the
Closed Loop method an electronic controller runs an algorithm that
utilizes feedback data from sensors within the system to vary the
output of the power supply to control the temperature.
Temperature controllers can have a single directional output
or a bidirectional output. A temperature controller that has a
single directional output can operate in Heating or Cooling mode.
Controllers with a single directional output are used in maintaining
a constant temperature within a system surrounded by a
relatively constant ambient temperature (i.e. refrigeration or hot
food storage). A temperature controller with a bidirectional output
can operate in Heating and Cooling mode. Controllers with a
bidirectional output are used for maintaining a constant tempera-
ture within a system surrounded by an ambient environment with
large temperature uctuations (i.e. back-up battery storage, climate
control).
Temperature controllers can also have two regulation modes:
thermostatic (On/Off) or proportional control. Thermostatic control-
lers operate by turning on the TEM in order to heat or cool to a setpoint. The set point temperature tolerance is dened by a hysteresis
range. Once the set point is achieved the controller shuts off the
TEM. When the control temperature changes to outside the hyster-
esis range the controller turns on power to the TEMs and restarts
the cooling or heating mode process. This cycle continues until the
controller is shut down. Thermostatic control is often used in climate
control and refrigeration, where a narrow temperature swing can be
tolerated.
Proportional controllers use proportional regulation to maintain a
constant temperature with no swing in the control temperature.
This is often accomplished by using a Proportional Integral
Derivative (PID) algorithm to determine the output value and a
Pulse Width Modulation (PWM) output to handle the physical
control. When using a controller with a PWM output, a capacitor
can be placed (electrically) across the output to lter the voltage
to the TEM. Proportional controllers are often used in heating and
cooling systems where the temperature must stay constant (with
no change) regardless of the ambient temperature, such as liquid
chiller systems used in medical diagnostics.
Regardless of the controller used, the easiest feedback parameter
to detect and measure is temperature. The sensors most commonlyused by temperature controllers are thermocouples, thermistors,
and RTDs. Depending on the system; one or more temperature
sensors may be used for the purpose of control. The temperature
sensor feedback is compared by the controller to a set point or
another temperature to determine the power supply output.
The temperature feedback sensor(s) will most likely be determined
by the controller specied. Some controllers even include a sensor
with purchase.
T / Tmax = 1 / (1+N2), where N is a percentage of
current ripple, expressed as a decimal. Laird Technologies
recommends no more than a 10% ripple.
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To begin selection of a TEM controller,consider the following questions:
1. What is the maximum voltage & current of TEM used in
application? (also needed for selecting a power supply)
2. Does the system need to Heat, Cool or Heat & Cool?
3. Can the system tolerate a temperature swing of 3C?
Once answered, the selection of the basic functions of a tempera-
ture controller can be identied. The controller selected needs to be
capable of handling the maximum voltage and current to properly
control the TEM and power fans.
If the answers to question 2 is Heat or Cool and the answer to
question 3 is Yes then the required controller is single directional
and thermostatic.
If the answers to question 2 is Heat or Cool and the answer to
question 3 is No then the required controller is single directional
and proportional.
If the answers to question 2 is Heat & Cool and the answer to
question 3 is Yes then the required controller is bidirectional and
thermostatic.
If the answers to question 2 is Heat & Cool and the answer to
question 3 is No then the required controller is bidirectional and
proportional.
TEM controllers also can accommodate more advanced options to
trip alarms, control fan speeds and interface remotely with PC or UI,
but these are beyond the scope of this handbook. However, some
basic questions to consider for TEM controller designs are:
1. What alarms/indicators are required for User Interface?
2. Does the controller need to interface with a PC?
3. Does the TEM controller provide fan control?
4. Does the temperature set point need to be changed
by the end user?
Other design considerations may exist and should be considered
during system level design.
Laird Technologies offers a variety of Closed Loop Temperature
Controllers. The controller offering includes single and bidirectional
output controllers that employ thermistor temperature sensor
feedback, fan controls, alarms, and a range of control algorithms
ranging from thermostatic (ON/OFF) to PID. Laird Technologiesalso has the ability to customize and design temperature control-
lers to meet unique application requirements. Consult with a Laird
Technologies Sales Engineer on available product offerings or
customized solutions that may t to your design criteria.
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Parameters Required forDevice SelectionThere are certain minimum specications that everyone must
answer before the selection of a thermoelectric module (TEM) can
begin. Specically there are three parameters that are required.
Two of these parameters are the temperatures that dene thegradient across the TEM. The third parameter is the total amount
of heat that must be pumped by the device.
The temperature gradient across the TEM, actual T is not the same
as the apparent, system level T. The difference between these two
Ts is often ignored, which results in an under-designed system.
The magnitude of the difference in Ts is largely dependent on the
thermal resistance of the heat exchangers that are used on the hot
or cold sides of the TEM.
Unfortunately, there are no Hard Rules that will accurately
dene these differences. Typical allowances for the hot side of
a system are:
1. nned forced air: 10 to 15C
2. free convection: 20 to 40C
3. liquid exchangers: 2 to 5C above liquid temperature
Since the heat ux densities on the cold side of the system are
considerably lower than those on the hot side, an allowance of
about 50% of the hot side gures (assuming similar types of heat
exchangers) can be used. It is good practice, to check the outputs
of the selection process to reassure that the heat sink design
parameters are reasonable.
The third parameter that must be identied for the selection pro-
cess, is the total heat to be pumped by the TEM. This is often the
most difcult number to estimate. To reduce the temperature
of an object, heat must be removed faster than heat enters it.
There are generally two broad classications of the heat that
must be removed from the device. The rst is the real, sensible or
active heat load. This is the load that is representative of what
wants to be done. This load could be the I2R load of an electrical
component, the load of dehumidifying air, or the load of cooling
objects. The other kind of load is often referred to as the passive
heat load. This is the load due to the fact that the object is cooler
than the surrounding environment. This load can be composed of
conduction and convection of the surrounding gas, leak through
insulation, conduction through wires, condensation of water, and in
some cases formation of ice. Regardless of the source of these pas-sive loads, they must not be ignored.
There are other things that may be very important to a specic
application, such as physical dimensions, input power limitations
or cost. Even though these are important, they are only secondary.
Laird Technologies approach to thermoelectric module selection/
recommendation utilizes a proprietary computer aided design
program called AZTECwhich selects an optimized thermoelec-
tric design from a given set of parameters: hot side temperature,
desired cold side temperature, and the total heat load to be pumped
over the actual T.
A checklist has been enclosed to assist with dening your appli-
cations existing conditions. If you should require any further
assistance please contact one of Laird Technologies sales engineers.
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Sealant OptionsMost applications operate in a room temperature environment
and cool to below dew point. As a result, moisture in the environ-
ment will condense onto the cold side heat exchanger and may
accumulate around mounting hardware and eventually penetrate
to the TEM. The presence of moisture will cause corrosion that willdegrade the useful life of a thermoelectric. Two perimeter seal-
ants are generally used because they provide moisture protection
against condensation, have high dielectric strength and low thermal
conductivity.
Silicone (RTV) is an all purpose sealant that exhibits good sealing
characteristics and retains its elastomeric properties over a wide
temperature range, -60 to 200C. The sealant is non-corrosive to
many chemicals and exhibits good electrical properties with low
thermal conductivity. It is suitable for high volume applications for
ease of use and is cost effective. However, over time it is impervious
to vapor migration that can actually trap small amounts of moistureinside the TEM once the vapor condenses. This may or may not be a
problem dependent on life expectancy of application and environ-
mental conditions.
Epoxy (EP) is an effective barrier to moisture that exhibits a useable
temperature range of -40 to 130C. When cured the material is
completely uni-cellular and therefore the moisture absorption is
negligible. The material exhibits a low dielectric constant, low coef-
cient of thermal expansion and low shrinkage. Epoxies are ideal
for applications requiring long life expectancies. However, applying
epoxy onto TEM can be cumbersome as multiple llers are required
to be mixed and working life tends to be short, which makes it more
difcult to automate for higher volume production runs.
It should be noted that since sealants come in contact with the top
and bottom ceramic, they act as a thermal paths and transfer heat.
The thermal conductivity of RTV and Epoxy is low, but it still can
diminish the cooling performance of a TEM by up to 10%. However,
it is necessary to specify for applications that maybe susceptible to
condensation.
Thermoelectric ArrayWiring multiple TEMs together is commonly referred to as a TE
array. The decision to wire TEMs in series or in parallel is primarily
based on available input power requirements. No additional per-
formance benet will be achieved by wire arrangement. TE arrays
are commonly used for higher heat pumping capacities and can be
more efcient than a single TEM by taking advantage of dissipating
heat over a larger surface area. When mounting a TE array onto a
heat exchanger, the recommended lapping tolerances are 0.025
mm for two TEMs and 0.0125mm for three or more. This is done
to maximize the thermal contact between the TEM and mating heat
exchangers.
One advantage of wiring a TE array in parallel versus in series is
that the entire TE array will not fail if one TEM has an open circuit.
This can be benecial for applications that require redundancy.
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Design/Selection ChecklistThe information requested below is vital to the design/selection of a thermoelectric device to achieve your desired performance.
Please attempt to dene as many of your applications existing conditions and limiting factors as possible.
(Please indicate units on all parameters.)
I. Ambient EnvironmentTemperature = ____________________
o Air
o Vacuum
o Other
II. Cold SpotTemperature: ______________
Size: ______________
Insulated? ___________Type:_____________Thickness: _____________
Desired Interface:
o Plate
o Fins
o Fluid Flow (parameters) ________________
o Other _______________
III. Heat Sinko Finned - Free Convection
o Finned - Forced Convection
o Liquid Cooled
Maximum Heat Sink Temp. _________________ -or-Heat Sink Rating (C/W) ___________________
IV. Heat Load at Cold Spot = ____________________(if applicable, above should include:)
Active:
I2R __________________
Passive:
Radiation= _________________
Convection= ________________
Insulation Losses= _________________
Conduction Losses= ________________(e.g. leads)
Transient Load= _________________(Mass - time)
V. Restrictions on Power Available (indicate most important)o Current: _________________
o Voltage: __________________
o Power: __________________
o No Restrictions
VI. Restrictions on Size: ___________________
VII. To ensure the most effective response:
Please provide a rough, dimensioned sketch of the application, indicating the
anticipated physical conguration and thermoelectric module placement.
Please print this form and ll in the blanksTelephone: 888 246-9050 Email: [email protected]
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Thermoelectric Multistage(Cascade) ModulesA multistage thermoelectric module should be used only when a
single stage module does not meet control temperature require-
ments. Figure 4 depicts two graphs: the rst shows the T vs.
Normalized Power input (Pin/Pmax) of single and multistage mod-ules. The second graphs shows the T vs. COP. COP is dened as
the amount of heat absorbed at the cold side of the TEM (in thermal
watts) divided by the input power (in electrical watts).
These gures should help identify when to consider cascades since
they portray the effectiveT range of the various stages. A two-
stage cascade should be considered somewhere between a T of
40C and 65C. Below a T of 40C, a single stage module may
be used, and a T above 65C may require a 3, 4 or even 5 stage
module.
Figure 4: Multistage TemperatureDifferential Graphs
There is another very signicant factor that must always be con-
sidered and that is cost. As the number of stages increase, so doesthe cost. Certain applications require a trade-off between COP and
cost. As with any other thermoelectric system, to begin the selection
process requires the denition of at least three parameters:
Tc Cold Side Temperature
Th Hot Side Temperature
Qc The amount of heat to be removed
(absorbed by the cooled surface of the TEM) (in watts)
Once T (Th - Tc) and the heat load have been dened, utilization of
Figure 4 will yield the number of stages that should be considered.
Knowing COP and Qc , input power can also be estimated. The
values listed in Figure 4 are theoretical maximums. Any device that
is actually manufactured will rarely achieve these maximums, but
should closely approach this value.
Laird Technologies offers a line of MS Series cascades though thereare no standard applications. Each need for a cascade is unique, so
too should be the device selected to ll the need. Laird Technologies
has developed a proprietary computer aided design selection tool
called Aztecto help select a device. The three parameters listed
are used as inputs to the programs. Other variables such as physical
size, and operating voltage or current can, within limits, be used to
make the nal selection. More than 40,000 different cascades can
be assembled utilizing available ceramic patterns. This allows near
custom design, at near standard prices. When the three param-
eters have been dened, please contact a Laird Technologies sales
engineer for assistance in cascade selection.
Typical Device PerformanceWhen PERFORMANCE vs. INPUT POWER is plotted for any thermo-
electric device, the resultant curve will appear as in gure 5 below.
Performance can be T (Th - Tc), heat pumped at the cold side (Qc ),
or as in most cases, a combination of these two parameters.
Input power can be current (I), voltage (V) or the product of IV.
When we refer to the Tmax or Qc max, we are referring to that
point where the curve peaks. The same is true when referring to
either Imax or Vmax. Since operating at or very near the peak is
relatively inefcient, most devices are operated somewhere
between 40% and 80% of Input Power MAX.
As stated, devices are normally operated on the near-linear,
upward sloping portion of the curve. When automatic or closed loop
temperature control is being used, current or voltage limits should
be set below the MAX intercepts.
Figure 5: Performance vs Input Power
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Assembly TipsThe techniques used in the assembly of a thermoelectric system
can be as important as the selection of the thermoelectric module
(TEM). It is imperative to keep in mind the purpose of the assembly
namely to transfer heat. Generally a TEM, in cooling mode, moves
heat from an object to ambient environment. All of the mechanical
interfaces between the device to be cooled and ambient are alsothermal interfaces. Similarly all thermal interfaces tend to inhibit the
transfer of heat or add thermal resistance to system, which lowers
COP. Again, when considering assembly techniques every reason-
able effort should be made to minimize the thermal resistance
between hot and cold surfaces.
Mechanical tolerances for heat exchanger surfaces should not
exceed .025 mm/mm with a maximum of .076 mm total Indicated
Reading. If it is necessary to use multiple TEMs in an array between
common plates, then the height variation between modules should
not exceed 0.025 mm (request tolerance lapped modules when
placing order). Most thermoelectric assemblies (TEAs) utilizethermal interface materials, such as grease. The grease thickness
should be kept to 0.025 .013 mm to minimize thermal resistance.
A printers ink roller and screen works well for maintaining grease
thickness. When these types of tolerances are to be held, a certain
level of cleanliness must be maintained to minimize contaminants.
Once the TEMs have been assembled between the heat exchangers,
some form of insulation should be used between the exchangers
surrounding the modules. Since the area within the module, (i.e. the
element matrix), is an open DC circuit and a temperature gradient
is present, air ow should be minimized to prevent condensation.
Typically, a TEM is about 5.0 mm thick, so any insulation that can
be provided will minimize heat loss between hot and cold side heat
exchangers. The presence of the insulation/seal also offers protec-
tion from outside contaminants.
The insulation/seal is often most easily provided by inserting a die
cut closed cell polyurethane foam around the cavity and sealing
with either an RTV type substance or, for more physical integrity, an
epoxy coat. Whatever form is used, it should provide the protection
outlined above. It is often desirable to provide strain relief for the
input lead wires to TEM, not only to protect the leads themselves,
but to help maintain the integrity of the seal about the modules.
We have included an Assembly Tips drawing (Fig. 6). This drawing
shows the details of the recommended construction of a typical
assembly. The use of a spacer block yields maximum heat trans-
fer, while separating the hottest and coldest parts of the system,
by the maximum amount of insulation. The spacer blocks are
used on the cold side of the system due to the lower heat ux
density. In addition, the details of a feed thru and vapor sealing
system that can be used for maximum protection from the
environment are shown.
If you follow the recommendations shown in these drawings than
you will see a signicant improvement in performance. When
testing an assembly of this type it is important to monitor tempera-
ture. Measuring temperature of the cooling uids, inlet and outlet
temperatures as well as ow rates is necessary. This is true if either
gas or liquid uids are used. Knowing input power to the TEM, both
voltage and current, will also help in determining the cause of a
potential problem.
In addition we have enclosed step-by-step procedure for assem-
bling Laird Technologies modules, Solderable or Lapped modules to
heat-exchangers.
If you should require any further assistance, please
contact one of our engineers. Our many years of
experience in working with customers ensuring reli-
able and efcient application of our products hasproven to be essential to product success.
Figure 6: Assembly Tips Drawing
Figure 7: Assembly Procedures Drawing
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Device Performance Formulae
Heat Pumped at Cold Surface: QC= 2N [aI T
C- ((I2r) / (2 G)) -kT G]
Voltage: V = 2N [((I r) / G) + (aT)]
Maximum Current: Imax
=(kG / a) [(1 + (2 Z TH)]1/2- 1]
Optimum Current: Iopt
= [kT G (1 + (1 + Z Tave
)1/2)] / (aTave
)
Optimum COP (calculated at Iopt
): COPopt
=(Tave
/ T) [((1 + Z Tave
)1/2 - 1) / ((1 + Z Tave
)1/2+ 1)] - 1/2
Maximum T with Q = 0 Tmax
= Th - [(1 + 2 Z TH)1/2- 1) / Z]
Notation Denition
TH Hot Side Temperature (Kelvin)
TC Cold Side Temperature (Kelvin)
T TH- T
C(Kelvin)
Tave
1/2 (TH+ T
C) (Kelvin)
G Area / Length of T.E. Element (cm)
N Number of Thermocouples
I Current (amps)
COP Coefcient of Performance (QC
/ IV)
a Seebeck Coefcient (volts / Kelvin)
r Resistivity (cm)
k Thermal Conductivity(watt / (cm Kelvin))
Z Figure of Merit(a2/ (rk)) (Kelvin-1)
S Device Seebeck Voltage(2 aN) (volts / Kelvin)
R Device Electrical Resistance(2 rN / G) (ohms)
K Device Thermal Conductance(2 kN G) (Watt / Kelvin)
Geometry Factor (G)
TEM G TEM G
OT 08 -xx- 05 0.016OT 12 -xx- 06 0.024
OT 15 -xx- 05 0.030
OT 20 -xx- 04 0.040
CP 08 -xx- 06 0.042
CP 08 -xx- 05 0.052
CP 10 -xx- 08 0.050
CP 10 -xx- 06 0.061
CP 10 -xx- 05 0.079
CP 14 -xx- 10 0.077
CP 14 -xx- 06 0.118
CP 14 -xx- 045 0.171
CP 20 -xx- 10 0.184
CP 20 -xx- 06 0.282
CP 28 -xx- 06 0.473
CP 5 -xx- 10 0.778CP 5 -xx- 06 1.196
PT 2 -12- 30 0.046
PT 3 -12- 30 0.057
PT 4 -12- 30 0.079
PT 4 -7- 30 0.076
PT 4 -12- 40 0.076
PT 6 -xx- xx 0.121
PT 8 -xx- xx 0.171
HT 2 -12- 30 0.046
HT 3 -12- 30 0.057
HT 4 -12- 30 0.079
HT 4 -7- 30 0.076
HT 4 -12- 40 0.076
HT 6 -xx- xx 0.121
Typical Material Parameters
T (Kelvin) r k Z 273 9.2 x 10-4 1.61 x 10-2 2.54 x 10-3
300 1.01 x 10-3 1.51 x 10-2 2.68 x 10-3
325 1.15 x 10-3 1.53 x 10-2 2.44 x 10-3
350 1.28 x 10-3 1.55 x 10-2 2.22 x 10-3
375 1.37 x 10-3 1.58 x 10-2 1.85 x 10-3
400 1.48 x 10-3 1.63 x 10-2 1.59 x 10-3
425 1.58 x 10-3 1.73 x 10-2 1.32 x 10-3
450 1.68 x 10-3 1.88 x 10-2 1.08 x 10-3
475 1.76 x 10-3 2.09 x 10-2 8.7 x 10-4
These tables and attributes are also available on AZTECthermoelectric module selection software
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Heat Transfer FormulaeNOTE:Due to the relatively complex nature of heat transfer, results gained from application of these formulae, while useful,
must be treated as approximations only. Design safety margins should be considered before nal selection of any device.
1) Heat gained or lost through the walls of an insulated container:
Where:Q = Heat (Watts)
A = External surface area of container (m2)
T = Temp. difference (inside vs. outside of container) (Kelvin)
K = Thermal conductivity of insulation (Watt / meter Kelvin)
X = Insulation thickness (m)
2) Time required to change the temperature of an object:
Where:t = Time interval (seconds)
m = Weight of the object (kg)
Cp= Specic heat of material (J / (kg K))
T = Temperature change of object (Kelvin)
Q = Heat added or removed (Watts)
NOTE:It should be remembered that thermoelectric devices do not add or remove heat at a constant rate when T is
changing. An approximation for average Q is:
3) Heat transferred to or from a surface by convection:
Where:Q = Heat (Watts)
h = Heat transfer coefcient (W / (m2K))
(1 to 30 = Free convection - gases, 10 to 100 = Forced convection - gases)
A = Exposed surface area (m2)
T = Surface Temperature - Ambient (Kelvin)
Conversions:
Thermal Conductivity 1 BTU / hr ft F = 1.73 W / m K 1 W / m K = 0.578 BTU / hr ft F
Power (heat ow rate) 1 W = 3.412 BTU / hr 1 BTU / hr = 0.293 W
Area 1 ft2= 0.093 m2
1 m2= 10.76 ft2
Length 1 ft = 0.305 m 1 m = 3.28 ft
Q = (A x T x K) / (X)
t = (m x Cp x T) / Q
Q = h x A x T
Qave = (Q (Tmax) + Q (Tmin)) / 2
Specic Heat 1 BTU / lb F = 4184 J / kg K 1 J / kg K = 2.39 x 10-4 BTU / lb F
Heat Transfer Coefcient 1 BTU / hr ft2F = 5.677 W / m2K 1 W / m2K = 0.176 BTU / hr ft2F
Mass 1 lb = 0.4536 kg 1 kg = 2.205 lb
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Air 1.2 0.026 1004
Alumina Ceramic-96% 3570 35.3 837 6.5
Aluminum Nitride Ceramic 3300 170-230 920 4.5
Aluminum 2710 204 900 22.5
Argon (Gas) 1.66 0.016 518
Bakelite 1280 0.23 1590 22.0
Beryllia Ceramic-99% 2880 230 1088 5.9
Bismuth Telluride 7530 1.5 544 13.0
Brass 8490 111 343 18.0
Bronze 8150 64 435 18.0
Concrete 2880 1.09 653 14.4
Constantan 8390 22.5 410 16.9
Copper 8960 386 385 16.7
Copper Tungsten 15650 180-200 385 6.5
Diamond 3 500 2300 509
Ethylene Glycol 1116 0.242 2385
Glass (Common) 2580 0.80 795 7
Glass Wool 200 0.040 670
Gold 1 9320 310 126 14.2
Graphite 1625 25-470 770 4.7
Iron (Cast) 7210 83 460 10.4
Kovar 8360 16.6 460 5.0
Lead 11210 35 130 29.3
Molybdenum 10240 142 251 4.9
Nickel 8910 90 448 11.9
Nitrogen (Gas) 1.14 0.026 1046
Platinum 21450 70.9 133 9.0
Plexiglass (Acrylic) 1410 0.26 1448 74
Polyurethane Foam 29 0.035 1130
Rubber 960 0.16 2009 72
Silicone (Undoped) 2330 144 712
Silver 10500 430 235
Solder (Tin/Lead) 9290 48 167 24.1
Stainless Steel 8010 1 3.8 460 17.1
Steel (Low Carbon) 7850 48 460 11.5
Styrofoam 29-56 .029 1.22 Teon 2200 0.35
Thermal Grease 2400 0.87 2093
Tin 7310 64 226 23.4
Titanium 4372 20.7 460 8.2
Water (@ 70F) 1000 0.61 4186
Wood (Oak) 610 0.15 2386 4.9
Wood (Pine) 510 0.11 2805 5.4
Zinc 7150 112 381 3 2.4
Material Density Thermal Specic Thermal Expansion
Name kg/m3 Conductivity Heat Coefcient x 10-6
W/m-K J/kg-K cm/cm/C
Typical Properties of Materials (@ 21C)
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Reliability & Mean TimeBetween Failures (MTBF)Thermoelectric devices are highly reliable due to their solid state
construction. Although reliability is application dependent, MTBFs
calculated as a result of tests performed by various customers
are on the order of 200,000 hours at room temperature. Elevatedtemperature (80C) MTBFs are conservatively reported to be on the
order of 100,000 hours. Field experience by hundreds of customers
representing more than 7,500,000 of our CP type modules and more
than 800,000 OptoTEC type modules during the last ten years
have resulted in a failure return of less than 0.1%. More than 90%
of all modules returned were found to be failures resulting from
mechanical abuse or overheating on the part of the customer.
Thus, less than one failure per 10,000 modules used in systems
could be suspect of product defect. Therefore, the combination
of proper handling, and proper assembly techniques will yield an
extremely reliable system.
Historical failure analysis has generally shown the cause of failure
as one of two types: Mechanical damage as a result of improper
handling or system assembly techniques.
Moisture:
Moisture must not penetrate into the thermoelectric module area.
The presence of moisture will cause an electro-corrosion that
will degrade the thermoelectric material, conductors and solders.
Moisture can also provide an electrical path to ground causing an
electrical short or hot side to cold side thermal short. A proper seal-
ing method or dry atmosphere can eliminate these problems.
Shock and Vibration:
Thermoelectric modules in various types of assemblies have for
years been used in different Military/Aerospace applications.
Thermoelectric devices have been successfully subjected to shock
and vibration requirements for aircraft, ordinance, space vehicles,
shipboard use and most other such systems. While a thermoelectric
device is quite strong in both tension and compression, it tends to
be relatively weak in shear. When in a severe shock or vibration
environment, care should be taken in the design of the assembly to
ensure compressive loading of thermoelectric modules.
Mechanical Mounting:
A common failure mode during assembly of a thermoelectric
module is un-even loading induced by improper torqing,
bolting patterns, and mechanical conditions of heat exchangers.
The polycrystalline thermoelectric material exhibits less strength
perpendicular to the length (growth axis) than the horizontal axis.
Thus, the thermoelectric elements are quite strong in compres-
sive strength and tend to be weak in the shear direction. During
assembly, un-even torquing or un-at heat exchangers can cause
severe shear forces. (See assembly instructions for proper mounting
techniques.)
Inadvertent Overheating of the Module:
The direct soldering process does result in temperature restriction
for operation or storage of the modules.
At temperatures above 80C two phenomena seriously reduce
useful life:
Above 80C copper diffusion into the thermoelements occurs due
to increasing solid solubility in the thermoelectric material and
increasing diffusion rate. At 100 - 110C the combined solubility
and diffusion rate could result in approximately 25% loss of device
performance within 100 hours.
Above 85C in the soldering process (using Bismuth-Tin Alloy) smal
amounts of selenium, tellurium, antimony and nickel are inherently
dissolved into the bismuth-tin solder. Although the melting point
of the base solder is 136C, the combined mixture of all elements
results in either a minute eutectic phase or a highly effective solid
state reaction occurring at above 85C that starts to delaminatethe ends of the thermoelements by physical penetration between
cleavage planes in the thermoelectric material. This results in a
mechanical failure of the interface.
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